This invention relates to signal strength indicators generally and, more particularly, to a diversity signal strength indicator particularly useable in diversity type radio reception systems for purposes of cellular site selection and the like.
Signal strength indicators have been used in mobile cellular communication systems and the like to determine the strength of incoming signals from mobile transceiver units. In such systems, the signal from a given mobile unit at those sites which are closest to the mobile unit are the strongest signals, and those further away are the weakest signals. A computer compares the relative strengths of the signals at the different sites and thereby determines the relative location of the mobile unit within the cellular system. Once the location is determined, the site or sites closest to the mobile unit are selected by the computer for receipt of messages from the base station and transmission of these messages back to the mobile unit. In this way, messages may be sent at the same frequency to different mobile units at the same time, so long as the mobile units are located at cells sufficiently spaced from one another to prevent crosstalk.
A difficulty encountered with such systems, particularly when operating in the ultra high frequency range, is caused by random signal cancellation. This signal cancellation results because of multiple signal paths which are created and destroyed in random fashion as the portable transmitter is moved. The effect of this cancellation is commonly known as "Rayleigh fading".
The problem of Rayleigh fading has been partly overcome through use of diversity reception and detection in which the incoming signals from the remote transceiver are received at two separate and spaced antennas at each site, or cell. There are several types of such diversity reception systems: maximal ratio combining diversity, equal gain combining diversity, switch diversity and selection diversity.
In the maximal ratio combining diversity system, the two incoming diversity signals at the two spaced antennas are first squared. They are then put in phase with one another, or co-phased, through a regenerative feedback circuit or the like. The two co-phased signals are then summed, or averaged, to produce an output signal proportional to the power of the incoming signals.
Equal gain combining diversity systems operate substantially the same as the maximal ratio systems described above, except that the incoming signals are not first squared. Accordingly, the output signal is proportional to the voltage of the incoming diversity signals and not to the power as in maximal ratio combining diversity systems.
The outputs from these two combiner types of diversity systems are generally applied to an FM demodulator or other similar circuit to obtain or derive the informational content from the composite signal produced by the diversity system. When such combining diversity systems are employed in a cellular-like communication system, a signal strength indicator needed for transmission site selection purposes could be operated from the composite output signal of the combiner circuitry. In such event, in a maximal ratio system, site selection would be based on only the square of the voltage of the diversity input signal at the antenna which is proportional to the power of the signal. In the equal gain diversity systems, on the other hand, site selection would be based on only the magnitude of the voltage of the diversity input signal at the antenna and not based on the power of the signal. This approach limits versatility and introduces inaccuracy for purposes of site selection.
Another disadvantage of using the combining signal as the input to a signal strength indicator in combining type diversity systems results simply because of the fact that the greater the amount of circuit operations performed on the received diversity input signal at the antennas before these signals are applied to the input of any site selection signal strength indicator, the greater the chance that such circuitry will introduce error. That is, the combining circuitry can introduce fluctuations in the input signal to the site selection signal indicator which are unrelated to fluctuation in true signal strength at the antenna. For instance, the co-phasing circuitry used in both maximal ratio and equal gain combining diversity systems often have nonlinear regions of operation.
Since the information of the signals is contained in their frequency modulation in FM systems, these errors in signal amplitude are not necessarily critical with respect to obtaining necessary minimal signal to noise ratios and for purposes of deriving the informational content of the frequency modulated signals. However, for purposes of determining signal strength to enable accurate transmission site selection, such amplitude errors are more troublesome.
Known techniques for transmission site selection in switch diversity and selection diversity reception systems also suffer from certain disadvantages. This is partly because the input to the signal strength indicators is taken from only one antenna at a time and is therefore subject to Rayleigh effects.
In switch diversity systems, the signal at only one of the plurality of antennas is monitored by a signal strength indicator of the type which produces a signal proportional to the logarithm of the incoming signal. If the signal strength indicator indicates a strength below a preselected minimum threshold level, the scan circuitry switches to another antenna until one is found having an associated signal strength which exceeds the minimum threshold level. Thus, the signal strength indicator receives an input signal from only one antenna at a time. Accordingly, the output of this signal strength indicator is sensitive to Rayleigh effects and therefore does not necessarily provide an accurate representation of the average strength of the signal received at the site. In switch selection diversity systems, a separate signal strength indicator for transmission site selection purposes could be fixedly connected to one of the plural antennas. However, such a signal strength indicator is also subject to Rayleigh effects.
In selection diversity systems, a separate conventional signal strength indicator is provided for each of a plurality of antennas at the site. The frequency demodulator or the like is switched to the antenna having the maximum signal strength as detected by scanning the plurality of signal strength indicators respectively associated with the plurality of antennas. If only a fixed one of these signal strength indicators is used as the source for transmission site selection, then it is, of course, subject to Rayleigh effects. If, on the other hand, the signal at the selected antenna having the signal of maximum strength is used as the signal for monitoring signal strength for site selection purposes, then again site selection will be subject to Rayleigh effects.
Another approach to signal strength measurement for site selection purposes in selection diversity systems could be to sum, or average, the outputs of two or more of the plural signal strength indicators. The conventional signal strength indicators produce an output signal which is proportional to the logarithm of the amplitude of the input signal. However, the sum of a plurality of logarithms of a plurality of numbers is not equal to the logarithm of the sum of the numbers. Accordingly, the output of such a summing, or averaging, circuit would also fail to provide an accurate representation of the average signal strength for site selection purposes.
Thus, it is seen that although diversity reception systems are known, the advantages of such systems have not been successfully employed in signal strength indicators generally or for purposes of site selection in cellular type radio networks even though multiple signals from multiple antennas have been available for such purpose in diversity type radio reception systems.